1. Field of the Invention
This invention relates to a process for making vitreous honeycomb structures and, more particularly, for making high purity vitreous honeycomb structures from silica soot by extrusion. These structures can be used by themselves or as preforms for consecutive processes such as viscous reforming operations.
While common structures made of glass have become inseparable from our daily lives, the unique properties of glass also allow its use in complex structures for high-technology specialty applications. Honeycomb structures comprising glass belong in the latter category. Glass honeycomb can be made with an array of channels and indeed have been made by other processes usually requiring fusing many individual glass tubes together. Structures made in such a manner are typically limited in channel diameter and homogeneity due to the fusing process. Structures of this type have also been made of other materials, but glass offers a combination of unique properties that allow such a structure to be hot drawn down and used in novel applications and technologies, especially where high surface area is required. An example of such a utility would be to facilitate or catalyze chemical reactions. Additional benefits derive from the high purity and/or high clarity and transparency that can be obtained from a glass honeycomb article. A honeycomb structure composed of glass is, therefore, an ideal article for supporting reactions requiring the initiation by actinic light.
Furthermore, because of the cell-like structure, glass honeycomb structure is extremely strong along the channel axis and yet is significantly lighter in weight than solid bulk glass. It is thus ideal for use as a support for such items as mirrors and the like by forming a sandwich construction (ref. CELLULAR SOLIDS, STRUCTURES, AND PROPERTIES, 2nd ed., Lorna J. Gibson and Michael F. Ashby, 1997). Glass honeycomb materials can have a significant benefit where weight is an issue as, for example, extraterrestrial payload. In one example, it is especially beneficial for supporting the reflecting surface of a mirror. The honeycomb support material additionally can be of similar low thermal expansion coefficient to the mirror material in order to prevent distortion or breakage due to thermal stress.
Silica soot is a by-product of the high purity fused silica glass making process. Until now, it has been considered a waste material that is typically discarded even though it is essentially pure silicon dioxide. The increasing demand for high purity fused silica exacerbates this waste problem. Therefore, there is a strong desire to reduce this wastestream both from an ecological as well as a financial perspective. Most advantageous would be to find a productive, commercial application for the material.
2. Prior Art
Conventional processes have been used to create glass honeycomb structures, but these differ considerably from the inventive process. The prior art approaches to manufacturing this type of glass honeycomb article are either to fuse individual hollow fibers or tubes together or to machine out a solid piece of glass to form a multi-channelled article.
These prior art processes are problematic for several reasons. Firstly, it is difficult to fuse multiple hollow fibers (i.e., fine capillary tubes) to form a multi-channelled article which can then optionally be hot-drawn down and rebundled again and again into a progressively finer and finer array of hollow channels. Secondly, it is difficult to assemble and fuse multiple hollow tubes uniformly into a perfect honeycomb structure. Thirdly, the diameter of the individual hollow fibers that can be easily handled limits the number of tubes in the first bundle towards making the honeycomb structure, because there is a practical limit to the diameter of the assembly that can be uniformly hot-drawn down. Lastly, it is extremely expensive and time consuming to machine a multitude of deep channels into a glass object.
Ceramic honeycomb structures such as Celcor(copyright) (a cordierite honeycomb structure used commercially as a substrate for automotive catalytic converters) have been paste-extruded from particulate material, but the resulting honeycomb article is not transparent to light, reducing its utility. In addition, such honeycomb article is crystalline in nature, preventing it from post forming such as hot-drawn down. Further, the particle size of the raw material used in the Celcor(copyright) process is approximately two orders of magnitude larger than the soot used in the present invention. The particle size can significantly affect the minimum web thickness for an extrudable honeycomb structure by direct extrusion.
It is therefore an objective of the present invention to provide a commercial application for high purity silica soot.
It is therefore another objective of the present invention to provide a commercial application for silica soot.
It is another object of the invention to provide a paste-extrusion and sintering process for the conversion of the silica soot into a glass article.
It is further an object of the invention to provide a glass honeycomb structure having high optical clarity and/or high UV transmission, coupled with good mechanical strength, and excellent thermal stability.
It is yet another object of the invention to utilize a glass honeycomb structure in such technologies as filtration, water purification, membrane reactors, flow controllers, bio-reactors, structural dielectric, and structural supports.
The current invention is designed to address the above-mentioned objects and prior art deficiencies. In particular, a process is disclosed for converting a silica containing vitreous powder, specifically a high purity fused silica (HPFS(copyright)) or a doped silica soot, into a glass honeycomb article. This is the first known process utilizing fused silica soot to generate such articles which themselves possess unique and advantageous properties.
Glass honeycomb structures can be made by adapting the conventional practice of honeycomb paste-extrusion to using vitreous powder as the starting material. Of special interest is the high silica content glass honeycomb structures made of high purity fused silica soot, and soot derived from silica and other metals, for example silica soot containing up to 9% titanium. High purity silica is defined as essentially pure silicon dioxide, trace materials may be present due to specific process or isolation techniques but these trace materials are considered contaminants adding no beneficial properties to the pure silicon dioxide.